Remove byte* property in ByteBufferAllocator (#5191)

* Remove byte* property in ByteBufferAllocator.

This allows consumers to read/write into native memory, but without
having to always pin the managed `byte[]` when working with managed
memory. This allows for users to not need to Dispose() ByteBuffers
when they are using the default ByteArrayAllocator class.

Instead, we use `Span<byte> GetSpan()` methods to get access to the
underlying memory buffer.

Fix #5181

* Add a set of benchmark tests.

* Add ReadOnly spans.

This allows consumers to use ReadOnlyMemory<byte> as the backing storage
for ByteBuffers, which is useful in read-only scenarios.

* Run tests using ENABLE_SPAN_T in appveyor.

* Fix FlatBuffers.Test.csproj to work on older MSBuild versions.

* Change the test script to test UNSAFE_BYTEBUFFER

* Address PR feedback.

Remove IDisposable from ByteBuffer.

* Respond to PR feedback.

tests/FlatBuffers.Benchmarks/FlatBuffers.Benchmarks.csproj

@@ -0,0 +1,21 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>netcoreapp2.1</TargetFramework>
+    <LangVersion>latest</LangVersion>
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+    <DefineConstants>$(DefineConstants);UNSAFE_BYTEBUFFER;BYTEBUFFER_NO_BOUNDS_CHECK;ENABLE_SPAN_T</DefineConstants>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="BenchmarkDotNet" Version="0.11.3" />
+    <PackageReference Include="BenchmarkDotNet.Diagnostics.Windows" Version="0.11.3" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Compile Include="..\..\net\FlatBuffers\*.cs" Link="FlatBuffers\%(FileName).cs" />
+    <Compile Include="..\MyGame\**\*.cs" Link="MyGame\Example\%(FileName).cs" />
+  </ItemGroup>
+
+</Project>